A. Field of the Invention
This invention relates to communication systems and apparatus, specifically, mobile radio telephones. More particularly, the invention relates to methods and apparatus for increasing the communication range of small, portable radio telephones.
B. Description of Background Art
Radio telephones permitting two-way voice communications are being used in increasing numbers in automobiles. The use of such mobile radio telephones, or "car phones", has accelerated with the installation of cellular communication networks in metropolitan areas. Cellular networks consist of a plurality of interconnected fixed transceivers. When an automobile equipped with a "cellular" radio telephone moves away from a fixed transceiver of the cellular network, the power level of the signals received by both the fixed transceiver and the mobile transceiver drops. When the power level of the signal received by a fixed transceiver site in the cellular network falls below a threshold value required for reliable communication, automatic switching circuitry in the network transfers or "hands off" communication between the mobile radio telephone and that fixed site to another cellular site closer to the automobile.
Each transceiver site of a cellular network employs a relatively low power transmitter. However, the practical two-way communication range of each fixed cellular transceiver site overlaps with the ranges of adjacent fixed transceivers. Thus, a plurality of fixed cellular sites with overlapping transmission ranges is capable of covering a wide geographic area, providing effective communication with mobile transceivers moving through the area.
In a cellular network, a relatively large number of mobile radio telephones may carry on separate two-way conversations, each on a different radio frequency carrier channel. However, to minimize the possibility that a signal transmitted on one channel might spill over into adjacent channels and thereby interfere with communications in adjacent channels, the transmitter power of mobile radio telephones is usually limited to a value of about 3 watts.
Radio telephones commonly installed in motor vehicles include a transceiver and antenna, and a separate handset similar in appearance to a conventional telephone handset. The handset typically includes a microphone, speaker, numeric key pad and display. Most transceivers capable of transmitting 3-watt signals tend to be heavier and bulkier than the handset. Thus, maximum power transceivers are oftentimes fixed underneath a car seat or installed in the trunk of an automobile, and connected by a coaxial RF power cable to an antenna mounted externally on the automobile. For such transceivers, the handset is also connected to the transceiver by a multi-conductor cable, allowing the handset to be located within easy reach of the driver. Usually, both ends of the cable used to interconnect the handset to the transceiver are provided with multiple pin plugs to facilitate installation of the radio telephone, and to permit the handset to be unplugged and stowed out of sight when the vehicle is unoccupied.
The microphone and speaker contained in the handset of a radio telephone perform the same functions as the corresponding elements of a conventional telephone handset. Thus, the microphone converts sound signals such as speech into electrical signals, which are then coupled to the transceiver and used to modulate a radio frequency carrier signal conveyed to the antenna. Also, modulated radio frequency signals received by the antenna are demodulated within the transceiver, and conveyed to the handset speaker where they are converted into intelligible sounds.
The keypad in the handset is used to dial out numbers, to answer calls, turn system power on and off, and to select operating modes of the system. Numbers dialed and function modes are displayed on the handset display unit.
In addition to the radio telephones described above, which are intended for relatively permanent or long-term installation in a vehicle, another type of radio telephone referred to as a transportable radio telephone or cellular phone exists. Transportable radio telephones contain internal batteries which permit the units to be removed from a vehicle and operated away from the vehicle. Typically, the RF power output of transportable radio telephones lies in the range of about 1.2 watts to 3 watts. The batteries required to operate transportable radio telephones at the aforementioned power levels make the units relatively bulky and heavy.
A third class of radio telephones, called hand-held cellular phones, exists. Hand-held cellular telephones are much more compact and lighter in weight than transportable cellular telephones, making the hand-held units easier to carry and use. However, the small size of hand-held units restricts their RF power output, usually to a level of approximately 0.6 watts. Therefore, the range of a hand-held radio telephones is substantially less than that of vehicle-mounted or transportable radio telephones, which operate at the maximum allowed RF power level of 3 watts.
Besides having a limited range, hand-held radio telephones are usually costly to manufacture, owing to the higher degree of micro-miniaturization required in their construction, as compared to larger vehicle-mounted or transportable radio telephones.
In view of the weight and size disadvantages of transportable radio telephones, and the power output limitations and high cost of hand-held radio telephones, the present invention was conceived of.